The first hearkens back to how important it is to have an editor. Other people see our work for what it is on the page. Not with all the assumptions and backhistory and everything else we have stored in our heads when we wrote it. Your editor (and your critique group) only sees what's there. What's there is what people read. Telepathy is not common enough for us writers to expect people to read our minds. Not to mention the fact that what's in our head rarely makes it intact onto the page.
So, you need an editor. Need, need, need. You need someone else to catch the typos, the odd turns of phrase, the times you wrote 'what' instead of 'that'. Because we see what we expect to see.
The second point comes from the habitual self-sabotage every writer performs. No sooner have we written the bloody thing than we start tearing it down. Part of that is the editing process, that vital, pernicious robber of joy. (in my opinion, alone, of course).
But the other part is the fact that what we can execute on isn't always what we conceived. In our minds, the characters were much more witty, more real. The drama had higher stakes, the emotions were more raw. When we look at what we actually committed to the page, we're disappointed. We see our own flaws too vividly. We remember the sentence we spent literally hours trying to fix. All of our turns of phrase feels precious and hollow on our ears. Sometimes we've spent so much time revising a piece that we hate it with the heat of a millions suns.